Transposable Genetic Elements. We are testing the possibility that transposition of genetic elements too small to be observed by conventional cytogenetic methods may play an important role in evolution and development. In order to study the possible role of transpositions and other sequence reorganization in evolution, we are using radiocation copy RNA from numerous cloned DNA segments from Drosophila melanogaster for hybridization in situ to the polytene chromosomes of melanogaster and its sibling species D. simulans and D. mauritiana. We find that unique sequences are generally located at the same sites in sibling species but that there are very major differences in the distribution of middle repetitive segments, a phenomenon we are now attempting to characterize in detail. We have also observed clear-cut differences between restriction digests of embryonic and various tissues of adult Drosophila melanogaster DNA. These differences are found on gel electrophoretograms of restriction digests, using radioactive cloned Drosophila DNA segments for hybridization according to the method of Southern (1975). We are attempting to determine whether the differences actually result from sequence rearrangement of instead from secondary modification or specific chain discontinuities. Nuclear RNA and DNA Sequence Organization. Heat shock induces invigorous transcription at a small number of chromosomal sites in Drosophila. We have used heat shock messenger RNA to select bacterial plasmids containing heat shock genes and adjacent DNA sequences. We are determining which restriction fragments from the cloned segments hybridize raddoactive nuclear RNA extracted from tissue culture cells at various times after temperature elevation. We will determine the pattern of transcription in the vicinity of several different heat shock genes cloned from D. melanogaster ad its sibling D. simulans, hoping to identify and study sequence elements that are involved in control and processing. We have also partially purified actin messenger RNA from Drosophila and will seek to obtain clones complementary to it in order to conduct similar studies of transcription and sequence organization.